Any, Nothing, and Unit in Kotlin

Photo by Mingwei Lim on Unsplash

In this short post, we’ll take a look at some of the special types in Kotlin. These types are Any, Nothing, and Unit. You may have used some or all of them but may not know the exact difference between them. If that’s the case then read on!

Any

The official documentation of Any states:

The root of the Kotlin class hierarchy. Every Kotlin class has Any as a superclass.

This means Any is the parent class of any class you define in Kotlin. This means if you follow the chain of parents of a class you will finally end up at Any. It’s the exact equivalent of Object in Java.

Here is the definition of Any. Based on the above paragraph and the following definition, every class in Kotlin has methods equals, hashCode and toString.

public open class Any {
    public open operator fun equals(other: Any?): Boolean

    public open fun hashCode(): Int

    public open fun toString(): String
}

Nothing

Based on the official documentation of Nothing

You can use Nothing to represent “a value that never exists”: for example, if a function has the return type of Nothing, it means that it never returns (always throws an exception).

Simply put we usually use Nothing as the return type of a function to indicate that the function always throws an exception.

Here is the definition of Nothing.

public class Nothing private constructor()

The above definition means that you cannot instantiate an instance of this class. It also means that no other class can inherit it.

An example usage ofNothing is in the error function which is part of the Kotlin standard library:

public inline fun error(message: Any): Nothing = throw IllegalStateException(message.toString())

As can be seen, because the error function always throws an exception, its return type is of type Nothing meaning that this function never completes successfully.

Unit

The definition of Unit is as follows:

public object Unit {
    override fun toString() = "kotlin.Unit"
}

Unit is the same thing as void in Java or C. When the return type of a function is Unit it means that the function does not return anything.

In Kotlin, the default return type of a function is Unit meaning that if you do not specify the return type of a function explicitly then the Kotlin compiler uses Unit as the return type.

That’s all!